The proposed work is an examination of the structure and acquisition of "mimetic depiction" in American Sign Language (ASL). ASL is a natural langage which has arisen independently of spoken languages within the visual-gestural modality. It has been claimed that the set of signs which we call "mimetic depiction" involve continuous, analogue variation; this is in sharp contrast to the discrete morphological distinctions universally found in spoken languages. If true, this claim would suggest that modality, for both spoken and signed languages, is an important determinant of language form. However, our own previous linguistic investigations have revealed that "mimetic depiction" signs are in fact not analogue in form; rather, like complex verbs in spoken languages, they are composed of a number of discrete morphemic components. The proposed research will begin by attempting to provide experimental support for this finding, and thereby for the view that modality is not a crucial constraint on morphological structure. It will then proceed to test an alternative view, namely that the acquisition process may itself make significant contributions to the structure of a language. This view is suggested not only by previous literature, but also by our own extensive pilot work, in which we have found that child learners of ASL are strongly biased toward analysing signs into sets of discrete morphological components. In fact, this basis is so strong that it appears to change the language across generations of learners. Our research will investigate these findings further by systematically testing various generations of signers and by studying the acquisition process in young deaf children across these generations.